Image sensor arrays typically include a two-dimensional array of pixels connected via gate lines and data lines. Each pixel includes a photo-sensitive cell, such as a photo-diode, for converting radiation (e.g., light or X-ray) into an electrical charge that may be stored and converted into an electrical signal transmitted via the data lines. As is well understood by those of ordinary skill in the art, each pixel may be switched on and off by a switching device, such as a transistor that is controlled via one of the gate lines. The switching device controls the charge read out of each photo-sensitive cell. For certain applications, the switching device in a pixel may be implemented using an amorphous silicon (“a-Si”) thin-film-transistor (“TFT”).
It has been observed that most common pixel switching devices, including a-Si TFT's, are sensitive to electro-static discharge (“ESD”). ESD events often occur during fabrication and later assembly of image sensor arrays. Therefore, ESD protection is needed for achieving yield and robustness in fabrication of image sensor arrays.
In image sensor arrays using TFT's, the threshold voltage of an a-Si TFT is known to shift value due to either charge trapping in the gate dielectric, and in the channel to dielectric interface. For a given TFT, since gate to drain resistance is extremely high, any external voltage applied on the gate remains indefinitely until it is drained through an external circuit. Trapped charge can further induce more defects in the intrinsic layer of a switching TFT, thus degrading performance. The threshold voltage shifting is affected by the voltage level and duration at the TFT gate. During normal operation, the threshold voltage shifting effect is not a problem even after many years of usage. But upon the occurrence of an ESD event, since the ESD voltage can exceed 100V, the TFT is susceptible to threshold voltage shifting. A typical symptom of an image sensor array that has incurred ESD damage is a darker or brighter line of pixels compared to neighboring lines of pixels in a dark image. The ESD affected lines may also suffer linearity problems during normal operation.
Typical prior art methods of protecting image sensor arrays from ESD damage include the use of ESD protection devices providing breakdown (i.e., shorting of current away from the device to be protected) at different threshold voltages. Such ESD protection devices include shorting-bars, and metal-insulator-metal (“MIM”) diodes. Many of the prior art solutions are not compatible with sensor testing and repairing requirements. Others are designed to work only for crystal silicon devices (a-Si is considered a slower mobility material), while others require additional fabrication steps.
One conventional solution for protecting an image sensor array from ESD damage is to use an external shorting bus to short gate and data lines together during the manufacture of the image sensor array. Near the end of the manufacturing process, the external shorting bus is removed. While this technique is useful for preventing ESD damage during the manufacturing process, it is not useful during normal operation of the image sensor array.
Some prior art techniques for designing and fabricating image sensor arrays have used on-substrate ESD protection structures in addition to, or in place of, an external shorting bus. Typically, one or more coupling devices are connected between an ESD-sensitive portion of the image sensor array (e.g., a pixel) and an ESD structure connected to ground or some other potential. The effectiveness of such protection schemes is dependent upon the ability of the coupling devices to provide: (1) a very low current leakage path between the sensitive portion of the image sensor array and the ESD structure in the absence of an ESD event; and (2) fast discharge of current from the sensitive portion of the image sensor array to the ESD structure upon the occurrence of an ESD event. While many attempts have been made to optimize these parameters, none have been very satisfactory.
Accordingly, it is an object of the present invention to provide an ESD system for use with image sensor arrays, the system providing very low current leakage in the absence of an ESD events, and very fast discharge of current upon the occurrence of an ESD event.
It is a further object of the present invention to provide an ESD protection system including a combination of ESD protection devices allowing for ESD protection across a wide range of ESD voltage levels.